Multifilaments are a completely different category of strings from monofilaments (like PSGD). The multis are constructed differently and are much more expensive to make because they are made from hundreds of strands of strings to simulate the playability of natural gut. Multis tend to be softer, have more feel and power but less durability.

PSGM is a multifilament. Multis are different from standard synthetic guts in that they are designed to be softer, add more comfort, and mimic natural gut. The best multis are the ones that play and feel closest to natural gut. Synthetic guts, despite what their name implies, do not come close to natural guts like multifilaments do in terms of softness and feel.

Basically you're looked at two very different strings here, and whichever one you prefer is a personal preference. If you're used to tournament nylon or already play with synthetic gut, PSGD is the best you can get for that price. If you play with multis or want to play with a softer string, PSGM is worth a try (although PSGM is not the best multi out there... Tecnifibre X-One Biphase and NRG2 and Wilson NXT are much better multis than PSGM to name a few).

It is also inexpensive as one, selling for $7.50 a package here at TW.

The above playtest includes the following summary comments:

"Synthetic Gut Multifilament is Prince's latest offering for the player looking for a playable multifilament string with a crisp feel and good durability, at a midrange price."

and:

"Prince Synthetic Gut Multifilament got great ratings from our playtest team. To date, our playtest program has tested 107 strings from different manufacturers. Any string that receives a score in the top five — as Prince Synthetic Gut Multifilament does for Control — is impressive. For that matter, any string that scores in the top 10 — as Prince Synthetic Gut Multifilament does for Playability — is impressive. In addition, though, Prince Synthetic Gut Multifilament scored well above average or better in every other category, too, which include Durability, Power, Touch/Feel, Comfort, Spin Potential, Tension Retention, and Resistance to Movement. It not only has a combined rating that is well above average, it is the 15th best string we've tested to date, in overall score. This is especially impressive considering its midrange price."

Prince's Original Synthetic Gut is the control string the USRSA uses in its playtesting. It was briefly retired when the widebody craze was upon the world, which is when PSGD was born and why, and then brought back once that fad dimmed too. POSG is still the better of their two synthetic guts, IMO.